In many cases, video data exists using an immense amount of data. Accordingly, when video data is transmitted from a transmission device to a reception device or when video data is stored in a storage unit, that video data receives compression coding.
As representative video coding standards, Moving Picture Experts Group phase 2 (MPEG-2), MPEG-4 and MPEG-4 Advanced Video Coding (MPEG-4 AVC/H.264) are known. These video coding standards are developed by International Organization for Standardization/International Electrotechnical Commission (ISO/IEC).
Also, as a new video coding statndard, High Efficiency Video Coding (HEVC, MPEG-H/H.265) is being developed (see non patent document 1 for example).
The above video coding stardards adopt two coding methods, i.e., the inter prediction coding and the intra prediction coding. The inter prediction coding is a coding method that uses information of a coded image in order to code a coding target image, while the intra prediction coding is a coding method that uses only information included in the coding target image in order to code the coding target image. A coding target image is also referred to as a frame or a picture.
Videos that are the targes of these video coding standards are natural images, obained mainly by using various tyeps of cameras. However, accompanying the progress in the information technology in recent years, a trend has emerged in which video coding is applied to screen content images such as those displayed in a personal compouter (PC) desk top etc. in additon to the application of video coding to natural images.
As a specific example of video coding of screen content images, there is a wireless display, which wirelessly transmits images displayed on a PC or a game device to a flat panel display device. As another specific example, there is Virtual Display Infrastructure (VDI), which transmits a user window of a virtual operating system to a mobile device using the Internet protocol.
Screen content images are artificial images generated by using computer graphics etc., and have characteristics different from those of natual images. The first characteristic of screen content image is low correlation of pixels. In natural images, the correlation is high between the pixel value of a target pixel and the pixel value of an adjacent pixel. Discrete Cosine Transform (DCT) adopted by all video coding standards utilizes this high correlation, and can realize high compression efficiency.
By contrast, a screen content iamge has many regions in which the correlation of pixel values is low. Examples of this inlcude the outlines of characters in a text edition window, a one-pixel-level narrow line in a Computer Aided Design (CAD) data edition window, and others. When DCT is applied to these regions, distortions of high-frequency components called mosquito distortions are perceived visually and remarkably particularly under a condition with a low bitrate.
In the first version of the HEVC, which was internationally standardized in 2013, a technique of conducting highly-efficient compression coding on screen content images is introduced partially. A specific example among others is Transform Skip (TS), which skips DCT applied for prediction errors in the intra prediction coding and the inter prediction coding.
Further, in the next version of HEVC, which is being standaized, a technique of increasing further the compression efficiency of screen content iamges is about to be introduced (see non patent document 2 for example).
A photography device that can generate a high-quality still iamge while suppressing an increase in the circut scale is also known (see patent document 1 for example). This photography device compresses a video so as to ouput the compressed video, expands the compressed video so as to output the expaneded video, generates a difference video between the expanded video and a video corresponding to the expanded video, compresses the difference video so as to output the compressed difference video. Thereafter, the photography device expands the compressed difference video so as to output the expanded difference video, adds the expanded video and the expanded difference video so as to generate an added video, and generates a still image by using the added video.
Patent document 1: Japanese Laid-open Patent Publication No. 2011-239240
Non patent document 1: ITU-T H.265ISO/IEC 23008-2, “High efficiency video coding”, 2013
Non patent document 2: D. Flynn et al., “High Efficiency Video Coding (HEVC) Range Extensions text specification: Draft 6”, JCTVC-P1005, 16th JCT-VC Meeting, San José, US, 9-17 Jan. 2014